To make the question more plastic: Do you want to reduce the size (footprint) of your (existing) database files or do you want to store data compressed, so it uses less storage? Of course, you can do both for an existing database, but you haven't told us much about it...

and want to know how to do it in 11g xe, as time was less , and i am new bie.

... where compression is one of the more advanced topics... Sorry to say, but reading and understanding the underlying concepts is necessary. Just providing you commands to do this or that might make you do something you may regret afterwards.